Today I tested a custom holder that has an optical glass (low iron glass) sheet held such that a negative taped to the bottom is at the same height as one sandwhiched between the standard Screen anti-newton glass sheet and the plastic anti-Newton holder. I used a 35mm technical pan negative scanned at 5300 spi. The short story is that the Screen AN sandwich holder was sharper by a little bit. I'll have to get out the micrometer and check the measurements, and I'll run another test with a different film, but it looks like good news, since avoiding Newton's rings with the glass holder would probably require wet-mounting.